FIG. 12 is a circuit diagram of a conventional amplifying circuit 100 with a bypassing function disclosed in JP2004-328400A. The amplifying circuit 100 includes an input port 101A connected with an antenna 101, a band-pass filter (BPF) 102 to which a signal is applied through the input port 100A from the antenna 101, an amplifier 103 to which an output signal of the BPF 102 is input, a switch 104 to which an output signal of the amplifier 103 is input, a bypass circuit 105 connected to the BPF 102 and the switch 104, and an output selector 106 for supplying a control signal to the switch 104 according to power of an output signal of the amplifier 103. The amplifier 103 includes an inverting amplifier 107 and a feedback circuit 108. The output signal of the switch 104 is supplied to an output port 100B of the amplifying circuit 100.
When power of the signal input to the amplifier 103 increases, the amplifier 103 output a signal with distortion, hence producing undesired noise and deteriorating a receiving quality. The output selector 106 detects the intensity of the output signal of the amplifier 103. According to the result of the detection, the output selector 106 determines whether or not the intensity of the signal input to the amplifier 103 is large enough to distort the output signal of the amplifier 103. Upon determining that the intensity of the signal input to the amplifier 103 is too large, the output selector 106 feeds supplies a control signal to the switch 104 to drive the switch 140 to connect between the output port 100B and the bypass circuit 105. The output selector 106 may control a feedback rate of the feedback circuit 108 to optimize the gain of the amplifier 103. These operations prevent the output signal of the amplifier 103 from having distortion.
In the conventional amplifying circuit 100, the switch 104 switches the signal to pass either the amplifier 103 or the bypass circuit 105. However, since the amplifier 103 and the bypass circuit 105 are different from each other in impedance characteristics, the profiles of propagation characteristics of the two routes of signal pass are different from each other. FIG. 13 illustrates a propagation profile of the signal passing from the antenna 101 to the switch 104 of the conventional amplifying circuit 100. The profile of the signal shown in FIG. 13 is a profile of S21 measured with a network analyzer having first and second ports. The first port is connected with a transmitting antenna. The second port is connected to amplifying circuit 100 with a bypassing function shown in FIG. 12. As shown in FIG. 13, the profile 110 of S21 represents the signal passing through the amplifier 103 while the profile 111 of S21 represents the signal passing through the bypass circuit 105. The amplifying circuit 100 with bypassing function is designed for receiving digital signals at frequencies ranging from 470 MHz to 750 MHz for a digital video broadcasting for a handheld (DVB-H) system. As shown in FIG. 13, the difference between the profiles 110 and 111 at 470 MHz is explicitly different from the difference between profiles 110 and 111 at 750 MHz. The profiles 110 and 111 are significantly different from each other at frequencies ranging from 470 MHz to 750 MHz. This difference of the profiles changes the amplitude of the signal at the output port according to the frequency of the signal before and after the switching of the switch 104. As shown in FIG. 13, the difference at about 750 MHz between the profiles 110 and 111 is about 15 dB while the difference at about 470 MHz between the profiles 110 and 111 is about 30 dB. A receiver for the DVB-H system often operates under an environment that creates frequency selective fading which causes a significant change in the intensity of the received signal by time. If the intensity of the received signal is significantly reduced by the frequency selective fading while the signal is initially received through the bypass circuit 105, the large difference between the profiles 110 and 111 deteriorates the output signal of the amplifying circuit 100 and changes a frequency characteristic of the amplifying circuit 100. The quality of the signal received by the receiver becomes low, and accordingly reduces communication quality.